Coherent optical receivers (COR) are being employed in modern fiber-optic links that utilize coherent optical communication, typically in the form of an integrated coherent receiver (ICR) wherein one or more optical mixers are tightly integrated with output photodetectors, often in a single chip. In order to guarantee a desired level of performance of a COR in a communication network, the receiver has to be extensively tested prior to installation with respect to a set of receiver parameters or characteristics. Receiver performance parameters that are typically measured include the Common Mode Rejection Ratio (CMRR), the group delay variation (GDV), the IQ skew, and the polarization skew.
The CMRR, which is an important parameter of coherent optical receivers, determines the capability of a coherent receiver to select one wavelength out of a number of alien wavelengths; the better the CMMR, the more alien wavelengths can be present without distortion of the communication signal carried by the target wavelength. Besides that, a good CMRR lowers the RIN (Relative Intensity Noise) requirements for an optical local oscillator.
The CMRR is a measure how symmetric the internal structures and photodiode responsivities of a ICR are manufactured, and may be defined as follows:
                    CMRR        =                  20          ⁢                                          ⁢                                    log              10                        ⁡                          (                                                                                          I                      1                                        ⁡                                          (                      f                      )                                                        -                                                            I                      2                                        ⁡                                          (                      f                      )                                                                                                                                  I                      1                                        ⁡                                          (                      f                      )                                                        +                                                            I                      2                                        ⁡                                          (                      f                      )                                                                                  )                                                          (        1        )            
where f is a frequency at which the CMRR is measured, and I1(f) and I2(f) are the electrical currents from two photodiodes that constitute an output differential detector of the ICR.
A typical ICR may include two input optical paths for two polarizations, which may include two optical mixers such as 90 deg optical hybrids, and differential detectors that include pairs of photodiodes followed by trans-impedance amplifiers (TIA) at the output. Accordingly, CMRR, which is a combined measure of optical and electrical imbalances in the ICR, may depend on non-idealities along optical paths, e.g. a non-ideal input optical splitting ratio (≠3 dB), inaccurate path differences, differing PD responsivities, TIA imbalances, and disbalances in front-end electronics such as bond wiring and electrical waveguides.
While measuring the CMRR for the continuous wave case, i.e. for f=0, is a relatively easy task, doing that for non-zero frequencies, e.g. in the RF range spanning megahertz (MHz) to tens of gigahertz (GHz) where the ICRs typically operate, is not trivial. The photodiodes in the IRC are typically wired such that only the differential photodiode component is connected to the output, so that the photodiode currents cannot be accessed individually, and the sum term in the denominator of equation (1) cannot be accessed directly. Therefore, the CMRR at the RF frequencies, i.e. RF-CMRR, is difficult to measure.
Another important parameter of a COR is the GDV. The GDV is a measure related to time distortion of a signal, and may be determined variation of the group delay of a signal in the COR with frequency. The group delay is a measure of the slope of the phase response at any given frequency, and is given by the following equation:
      τ    g    =            d      ⁢                          ⁢      Φ      ⁢                          ⁢              (        ω        )                    d      ⁢                          ⁢      ω      
However, the GDV may also be difficult to measure in integrated photonic devices based solely on the device output, without access to internal measuring points in the device.
Accordingly, it may be understood that there may be significant problems and shortcomings associated with current solutions and technologies for testing and characterizing coherent optical receivers, including integrated coherent receivers that are used in coherent optical communications.